Metal-air electrochemical cells, especially those wherein the metal is powdered zinc, have become increasingly popular power sources for small electrical devices. Metal-air cells have an inherent advantage over most other electrochemical cell systems in that for a given cell volume, metal-air cells have a greater capacity. The greater capacity is due to the fact that in metal-air electrochemical systems, oxygen from the atmosphere, which is essentially limitless, is the active cathode material. Hence, metal-air electrochemical cells do not contain consumable cathode material and, therefore, can contain a greater amount of anodic material. It is this increase in the amount of anodic material which leads to the increased, per unit volume, capacity of metal-air cells. Due to their high capacity and relatively flat discharge curve, metal-air cells are particularly adapted for use in those applications which require moderate drains and continuous discharge usage.
In metal-air cells, air containing oxygen, the cathodic reagent, enters the cell through port(s) in the cell can which are immediately adjacent to a cathode assembly. The air diffuses into an air cathode subassembly where the oxygen is reacted. This air cathode subassembly generally consists of mixtures of activating chemicals supported by a complex physical structure. The air cathode subassembly also slows the rate of diffusion of other gases, particularly carbon dioxide and water vapor, through the electrode to the reaction site. These gases in air, particularly water vapor, can have a profound limiting effect on the capacity of the cell.
Once the oxygen has entered the cell, it diffuses through a separator, which is a moisture barrier usually of a plastic-like material impervious to liquids such as the alkaline electrolyte, and reacts with the water in the electrolyte. This reaction consumes electrons and produces hydroxide ions which, after migrating into the anode chamber, oxidize the metal anode, generally producing two electrons for each atom of the metal reacted. Electrochemical cells comprised of metal anodes and air cathodes are well known, and are more fully discussed in references such as U.S. Pat. Nos. 3,149,900 (Elmore and Tanner) and U.S. Pat. No. 3,276,909 (Moos).
A major problem associated with metal-air electrochemical cells is the loss of cell capacity as a result of storage, shipping, etc. of the cell between the time the cell is manufactured and the time the cell is used as a source of electrical power. Another often-noticed limitation is the depressed open circuit voltage of such cells upon placement into service after storage, often of only a few weeks duration. The problems and limitations observed with metal-air electrochemical cells stem from the same factor which provides for their capacity advantage: interaction with the environment. Since the diffusion of oxygen into the cells begins a series of reactions which ultimately consume the anodic material, it is readily apparent that a significant ingress of oxygen into a metal-air electrochemical cell during storage will significantly reduce a cell's capacity, therefore reducing the viable shelf life for such an electrochemical cell. However, the ingress or egress of water vapor during storage can have an even more dramatic effect on the performance of metal-air cells after storage of even a few months.
Water is present in metal-air electrochemical cells since the electrolytes in such cells are aqueous alkaline solutions. And since the water in the electrolyte is directly involved in the reactions which produce the electric energy, any reduction in the water content of the cell due to the egress of water vapor attributable to a lower relative humidity in the external cell environment will decrease the reaction rate, i.e., the production of electrons. Such a decrease in the reaction rate necessarily reduces the rate capability and capacity of the cell. The ingress of water vapor, due to a higher relative humidity outside of the cell can have a similar deleterious effect on cell performance, since the cell becomes overfilled with water. The excess of water causes the premature conclusion of the electrochemical reactions and substantially reduces the rate capability of the cell.
In order to diminish the deleterious effects of the environment on metal-air cells, the air entry ports of metal-air cells are normally sealed with removable tabs (or tapes) upon manufacture. The removal of such a seal tab when a cell is placed in service theoretically ensures that the freshly unsealed cell has the approximate capacity of a freshly manufactured cell. Unfortunately, such theoretical fresh cell capacity has been difficult to consistently obtain, since the sealing means heretofore commercially used in the manufacture of metal-air cells have been unable to eliminate the recognized effects of the environment which occur during the storage of metal-air cells.
Presently, the air entry ports of most metal-air cells are sealed upon manufacture by tabs consisting of rubber based adhesives applied to a rubber impregnated paper face stock and overlayed with a polyester film. Metal-air cells sealed with such tabs display substantial reductions in cell capacity upon being placed into service as a source of electrical power after storage. Moreover, such cell tabs exhibit tape delamination, i.e., upon storage for long periods of time and/or at elevated temperatures, the strength of adhesive-to-cell case bond increases to the point where it exceeds the cohesive strength of the paper. When this phenomena occurs, upon removal of the tab, the adhesive and a layer of paper often remain on the cell case. Along with a decrease in cosmetic appeal, such cells often cannot be fully activated and may insulate the cell from electrical contact, thereby allowing for the possible perceived failure of the cell by the consumer.
Another type of cell sealing means, which uses rubber-based adhesives applied directly to polyester film, have been utilized to prevent the loss of cell capacity during storage of the unused cells. Such impervious tapes are quite effective in sealing off the cell from the environment. However, upon only a few weeks storage, the voltage of a metal-air electrochemical cell sealed with such a tape drops to the voltage of the metal-carbon couple, which for metal-air cells having a powdered zinc anode is 0.4 volts. This low voltage results from the insufficient ingress of oxygen to maintain the cell voltage. A consumer, upon removing such a tape from a metal-air cell may have to wait a considerable time before the functional voltage is re-established. In some cases, a consumer may perceive that the cell is defective.
Because of the aforementioned advantages of metal-air electrochemical cells, it is imperative that the environmental effects heretofore incumbent with the storage of metal-air cells be eliminated, without so isolating the cell from the environment such that the open circuit voltage upon placing the cell in service is unacceptable. Therefore, it is an objective of the present invention to provide a removable seal for a metal-air electrochemical cell which allows for the storage of such cells without the attendant decrease in cell performance.
Another objective of the present invention is to provide a removable seal for a metal-air electrochemical cell which reduces the diffusion of water vapor into or out of such cells during storage even under dry conditions at elevated temperatures.
Another objective of the present invention is to provide a removable seal for a metal-air electrochemical cell which allows the open circuit voltage of such cells upon placement in use after storage to have a functional open circuit voltage upon removal of the tab.
These and other subsidiary objectives which will appear are achieved by the practice of the present invention.